Hoping for any help or advice: we use MS Exchange 2003 and over the last 6 months some people sending mail to mainly two email addresses get mail delivery failure messages from the postmaster at our domain:

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.

      mspop3connector.<email address>@<our domain>

If they persist, the message usually comes through I believe.  The problem is erratic - so we may go all week without issue.  Any ideas gratefully received!